1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, or a fax machine using an electrophotographic system, and a developing device used therein.
2. Description of the Related Art
A dry developing method applied to an electrophotographic system includes a one-component developing method using only a toner and a two-component developing method using a developer including a toner and a magnetic carrier.
Since the one-component developing method does not include a magnetic carrier, an electrostatic image of an image bearing member is not disturbed by a magnetic brush formed of a magnetic carrier, but is suitable for enhancing an image quality. However, since the one-component developing method cannot stably impart a charge to the toner, the one-component developing method has a problem in stability of an image quality. Furthermore, since the one-component developing method does not include a medium for carrying a toner, like a magnetic carrier, the one-component developing method cannot impart a uniform carrying force to the toner, and a mechanical load to the toner may be easily increased when the toner is carried. Thus, the stability of the image quality may be easily reduced by deterioration of the toner.
On the other hand, although the two-component developing method has a problem in image quality, the two-component developing method may easily impart a charge to a toner. Furthermore, since a load to the toner is small, the stability of an image quality is high.
As a method for solving the problems of the above-described developing methods, a hybrid developing method is disclosed in Japanese Patent Laid-Open No. 9-211970. The hybrid developing method applies a carrying bias between a carrying roller (developer bearing member) for carrying a two-component developer and a developing roller (toner bearing member), covers the developing roller with a toner layer, and develops an electrostatic image of a photoreceptor (image bearing member) using the toner layer, thereby forming an image.
However, it is known that the hybrid developing method cannot stably cover the developing roller with a toner layer over a long period. The hybrid developing method covers the developing roller with a toner having a predetermined charge quantity Q/S, in order to bridge a potential difference ΔV which is generated between the carrying roller and the developing roller by the above-described carrying bias. At this time, the potential difference ΔV and the charge quantity Q/S of the toner per unit area to be covered are proportional to each other. Furthermore, the charge quantity Q/S corresponds to a product of the mass of toner related to covering per unit area (M/S) and a charge quantity of the toner per unit mass (Q/M). Thus, the following equation is established:ΔV∝Q/S=(M/S)×(Q/M)  Equation (1)
That is, in the hybrid developing method, the mass M/S of the toner related to covering per unit area is determined from the potential difference ΔV and the charge quantity Q/M of the toner per unit mass. Thus, the hybrid developing method has a problem in that, when the charging amount of the toner is changed, a toner amount related to covering is varied according to the change in charging amount of the toner.
In order to solve such a problem, Japanese Patent Laid-Open No. 2009-8834 discloses a method for measuring the thickness of a toner layer on a developing roller using a toner layer thickness sensing member, when covering the developing roller with a toner layer. Furthermore, Japanese Patent Laid-Open No. 2009-8834 also discloses a method for controlling the thickness of the toner layer on the developing roller to a predetermined thickness by changing a carrying bias between the developing roller and the magnetic roller (developer bearing member) or the rotation numbers of the developing roller and the magnetic roller, based on the thickness of the toner layer.
However, since the method uses a toner density sensor or surface potential sensor as the toner layer thickness sensing member, the method may increase the size of a device or the cost. Furthermore, when the carrying bias or the rotation number of the developing roller is changed even in case where the thickness of the toner layer is controlled through the sensing member, a development condition between the photoreceptor and the developing roller in the downstream needs to be controlled at the same time. Thus, the control operation becomes complex. As a result, the method cannot accomplish the original goal that is stabilizing the toner mount on the photoreceptor.
As a developing method for stably forming a toner layer, Japanese Patent Laid-Open No. 10-198161 discloses a developing device using a rotatable regulating sleeve (developer regulating member) arranged at a predetermined interval from the developing roller. The developing device can stably impart a charge to a toner through a carrier, and cover a developing roller with a toner layer while preventing reduction in the density of an output image or occurrence of toner scattering. The developing device 320 is provided with a developing container 321 which contains a developer 310 including a toner and a magnetic carrier.
Hereinafter, the developing device 320 will be described with reference to FIG. 22.
The developing container 321 formed at a position facing a photoreceptor 301 has an opening in which a developing roller 322 and a developer collection member 323 are arranged. The developing roller 322 can be rotated in an arrow direction of FIG. 22, and the developer collection member 323 is positioned above the developing roller 322, with a predetermined interval provided therebetween. The developer collection member 323 includes a regulating sleeve 331 formed of a nonmagnetic material and a permanent magnet 332 fixed and arranged therein. The regulating sleeve 331 is rotatably supported in the same direction as the rotational direction (arrow direction of FIG. 22) of the developing roller 322. Furthermore, the developing container 321 includes a carrying member 324 which stirs a developer within the developing container 321 and supplies the developer to the developing roller 322, while rotating in the arrow direction of FIG. 22.
Next, a process of covering the developing roller 322 with a toner layer in the developing device 320 will be described.
The developer 310 within the developing container 321 is stirred by the carrying member 324 and supplied onto the developing roller 322. The supplied developer 310 is borne and carried onto the developing roller 322 magnetized by receiving a magnetic force of the permanent magnet 332 within the regulating sleeve 331, and regulated in a developer regulation region G.
FIG. 23 is an enlarged view of the developer regulation region G.
The magnetic carrier within the developer restrained by a magnetic field in the developer regulation region G is restrained by the magnetic force of the permanent magnet 332. Since the regulating sleeve 331 is rotated in an arrow direction of FIG. 23, the magnetic carrier receives a carrying force in a direction A of FIG. 23, in which the magnetic carrier is returned into the developing container 321, according to the rotation. Thus, while the magnetic carrier is restrained in the developer regulation region G, the magnetic carrier is sequentially returned into the developing container 321 by the carrying force from the regulating sleeve 331. Thus, the magnetic carrier does not leak to a developing portion facing the photoreceptor 301.
On the other hand, a nonmagnetic toner 311 within the developer in the developer regulation region G is not restrained by the magnetic field in the developer regulation region G. Furthermore, the nonmagnetic toner 311 adheres to the developing roller 322 due to a reflection force caused by a charge imparted through frictional charging between the magnetic carrier and the surface of the developing roller 322. Thus, the nonmagnetic toner 311 receives a carrying force in the rotational direction of the developing roller 322 (direction B of FIG. 23) according to the rotation of the developing roller 322, and passes through the developer within the developer regulation region G so as to cover the developing roller 322.
As described above, the magnetic carrier can cover the developing roller 322 only with the nonmagnetic toner to which a sufficient amount of charge is imparted, without leaking to the developing portion. The developing device disclosed in Japanese Patent Laid-Open 10-198161 uses a force acting on the toner which can be physically contacted with the developing roller. Thus, the developing device may prevent a phenomenon which is seen in the hybrid developing method, that is, a rapid variation in the toner amount related to covering due to a variation in charge quantity Q/M of the toner.
When the charge quantity of the toner is reduced, the hybrid developing method increases the toner amount related to covering. However, the developing device disclosed in Japanese Patent Laid-Open 10-198161 can suppress a variation in image density, which increases the toner amount, because the increase in toner amount related to covering is suppressed.
However, according to a detailed examination of the present inventor, the image uniformity of the developing device disclosed in Japanese Patent Laid-Open 10-198161 needs to be further improved, while a variation of image density is further suppressed.
FIG. 24 is a conceptual view of a toner layer which is obtained by the developing device 320 so as to cover the developing roller. In FIG. 24, a black portion indicates a part of the toner layer covering the developing roller, and a white portion indicates a region which is not covered with the toner layer. As illustrated in FIG. 24, regions which are not covered with the toner layer irregularly exist substantially in parallel to the rotational direction of the developing roller, and the density of the toner on the developing roller is not uniform. As such, when the covering layer of toner on the developing roller is non-uniformly formed, the image density may be easily reduced. That is because the area of white portions on a sheet, which are not covered with the toner, is increased during fixation and the image density is rapidly reduced.
The image density can be increased by adjusting the circumferential velocity of the developing roller and the photoreceptor and excessively supplying toner onto the photoreceptor. Specifically, the image density can be increased by further raising the circumferential velocity of the developing roller than the photoreceptor, when the developing roller and the photoreceptor are rotated in the same direction at facing portions thereof. Alternatively, the image density can be increased by setting the rotational directions of the developing roller and the photoreceptor to the opposite direction at the facing portions thereof. However, although a desired image density is obtained, in-plane density unevenness stands out as illustrated in FIG. 25B. In this case, an image having low image uniformity is inevitably obtained. Furthermore, from the viewpoint of reduction in energy consumption, the toner may be consumed more than necessary, while a desired image is required to be outputted at a smaller toner amount.
FIG. 25A is a schematic view illustrating a case in which an electrostatic image on the photoreceptor is ideally developed through a toner. FIG. 25B is a schematic view illustrating a case in which an image density is obtained through the above-described method.
Referring to FIG. 25A, a toner image having a high level of uniformity is obtained at a small toner amount. On the other hand, referring to FIG. 25B, however, a toner image having a low level of uniformity is obtained at large toner amount.
As the result of the detailed examination of the present inventor, the reason of such phenomenon can be described using the following model. This will be described with reference to FIG. 26.
FIG. 26 illustrates that the developer 310 carried in the rotational direction h of the developing roller 322 in the developer regulation region G forms magnetic brushes due to the magnetic field, and is restrained by the developer collection member 323 and carried in the rotational direction j of the developer collection member 323. In reality, a plurality of developers (not illustrated) exists as magnetic brushes.
While the developer 310 is carried over the developing roller 322, the toner 311 of the developer 310 is charged by coming in contact with the developing roller 322. At this time, the toner 311 is desorbed from the magnetic carrier 312, and adheres to the developing roller 322.
As described above, the developer 310 restrained by the developer collection member 323 is carried in the rotational direction j from the downstream of the rotational direction h. Since the developer 310 already consumed the toner 311 in the upstream of the rotational direction j, the magnetic carrier 312 within the developer 310 has an ability of collecting a toner. Thus, when the developer 310 carried in the rotational direction j of the developer collection member 323 comes in contact with the toner 311 adhering to the developing roller 322, the toner 311 is collected by the magnetic carrier 312, and returned into the developing container 321.
FIGS. 27A and 27B are schematic views illustrating that the toner 311 adhering to the developing roller 322 is collected by the magnetic carrier 312 of the developer 310.
When the developer 310 collides with the toner 311 on the developing roller 322 (FIG. 27A), a couple of forces act on the toner 311, and rotates the toner on the developing roller 322 (FIG. 27B). Thus, the adhering force between the toner and the developing roller decreases. At this time, since the magnetic carrier 312 is electrically charged at the opposite-polarity by the charge of the consumed toner, the toner covering the developing roller is scraped by the magnetic carrier 312 while passing through the developer regulation region G. As such, since a scraping trace is formed in the carrying direction of the developer 310, that is, substantially in parallel to the rotational direction of the developing roller or the developer collection member by the magnetic carrier, a uniform toner layer cannot be formed on the developing roller.